In some of conventional internal combustion engines equipped with a fuel injection mechanism, the fuel supply to all of the cylinders of the engine is cut off upon deceleration until the rotational speed of the crankshaft of the engine falls below a predetermined value such as 1,300 r.p.m. inasmush as engine output is not required when the throttle valve of the engine is fully closed. This cut-off of fuel supply results in effective engine braking and improvement of its fuel consumption characteristic. In such an engine, the fuel supply is reestablished when the rotational speed of the engine crankshaft falls below the predetermined value in order to prevent engine stall. According to the above-mentioned apparatus, since all of the cylinders are enabled (fueled) or disabled (non-fueled) at once depending on whether the rotational speed is above or below the predetermined value, the engine produces an impact or shock which will have an effect on the vehicle body. It will be understood that such an impact or shock is uncomfortable for the vehicle occupants.
Furthermore, the predetermined value at which the reactivation of the engine cylinders takes place has to be set at a relatively high value in order to prevent engine stall. However, this predetermined value is preferably as low as possible to improve fuel economy.
The inventors of the present invention had invented an apparatus for controlling the number of enabled cylinders upon decelerating for eliminating the drawbacks and disadvantages inherent to the above-mentioned conventional apparatus before they invented the present invention. The above-mentioned invention invented prior to the present invention will be referred to as prior invention. Although the prior invention is disclosed in a patent application (53-56892) filed with the Japanese Patent Office, the specification has not yet been published. To discuss the objects of the present invention, the technique in the prior invention will be briefly described hereinafter since the subject matter of the present invention resides in the improvement of the apparatus according to the prior invention.
The apparatus according to the prior invention comprises a plurality of comparators responsive to a signal indicative of the engine rotational speed upon deceleration of the engine. The threshold voltages of the comparators are arranged stepwise so that each comparator produces an output signal when the engine speed falls below each threshold voltage. The output signals of the comparators are supplied to logic circuits to control a plurality of switches via which a fuel injection control pulse signal is respectively applied to fuel injection valves to stepwise increase the number of enabled cylinders thereby preventing occurrence of impacts or shocks in the transition period of reactivation of the cylinders upon deceleration.
Assuming that the engine is mounted on a motor vehicle as the prime mover thereof, if the vehicle is not equipped with any auxiliary power consuming units driven by the engine, such as an air conditioner, the apparatus according to the prior invention satisfactorily operates. However, a large number of motor vehicles are actually equipped with auxiliary power consuming units. As is well known, when an air conditioner for a motor vehicle is turned on, the compressor of the air conditioner requires relatively high power so that the engine is needed to feed sufficient power to the compressor. Although the engine may supply sufficient power to the compressor when the engine operates at high speeds, when the number of enabled cylinders is made less than the maximum number of the total cylinders during low speed operation by means of the apparatus according to the prior invention, the engine cannot produce sufficient power that the compressor requires.
Usually, when a motor vehicle is equipped with an auxiliary power consuming unit such as an air conditioner, the engine rotational speed during idling is raised by increasing the flow rate of the intake air to prevent engine stall. Although this technique of elevation of the idling speed is appreciated, the engine still suffers from unstable operation and/or tendency of engine stall during the transition period of the stepwise reactivation of the cylinders since the engine power produced by the enabled cylinders less than the maximum number cannot afford the required power.
This imbalance between the engine power and the required power causes the engine to undesirably vibrate while the engine is still able to operate. Consequently, uncomfortable vibrations are emitted from the engine when the engine produces less power than required during the stepwise reactivation of the cylinders. This phenomenon of production of the undesirable vibrations continuously lasts until all of the cylinders are enabled.